1. Field of the Invention
The present invention relates to building materials, and particularly to a process using multiple waste streams to manufacture synthetic lightweight aggregate that can be used to make concrete and other building materials.
2. Description of the Related Art
The construction industry is very demanding on natural raw materials, especially concrete. Concrete production consumes vast quantities of aggregates. Such consumption can negatively impact the growth of local economies and infrastructures that rely on these aggregates to help sustain these endeavors by building new structures for business and personal property and maintenance on existing roads and buildings. Even more unfortunate, some areas of the world have scarce local aggregate sources to adequately support such activities. For example, Kuwait, a strong oil-based economic nation, has completely stopped quarrying for coarse aggregates. The construction industry there is now entirely dependent on imported aggregates from neighboring countries. Therefore, there is a need for alternative sources.
Manufactured or synthetic aggregates are examples of some substitutes for natural aggregates, especially as lightweight aggregates, hereinafter referred to as LWAs. Synthetic aggregates are usually characterized by their low density. According to ASTM specifications (C 330-05, 1989; C 331-05, 1994; and C 332-05, 1994), the bulk density of lightweight aggregates used in structural concrete, heat-insulating concrete, and concrete masonry units should be in the range of 0.88 to 1.12 g/cm3.
Synthetic aggregates are generally produced from a number of raw materials including clay, shale, slate, perlite, vermiculite, blast furnace slag, and pulverized fuel ash. The production of synthetic aggregate from clay involves heating suitable raw materials at a sufficiently high temperature so that it melts to a viscous, pyroplastic mass. The entrapped gases in the viscous mass cause expansion or bloating of the mass, and subsequently results in a porous structure with low density upon cooling.
Studies by Riley (1951) have shown that the chemical composition of unfired raw material indicates whether or not it can develop the proper viscosity at the melting point needed for gas entrapment, and thus reduce bloating. His research showed experimentally that the viscosity requirement can be satisfied if the chemical composition of the raw material is such that there is a proper ratio of fluxing oxides (CaO, MgO, FeO, Fe2O3, Na2O and K2O) to silica (SiO2) and alumina (Al2O3). The compositional relationship for satisfying the viscosity requirement in clays is demonstrated in a Riley triangle or diagram, an example of which is shown in FIG. 1.
One example of a synthetic aggregate involves mixtures of marine clay and CaF2-rich semiconductor-industry sludge in several clay-to-sludge ratios or loadings, namely 90/10, 70/30 and 50/50. The mixtures were fired in a bench-scale rotary kiln. The results produced synthetic aggregates that exhibited good bloating during firing and low densities, as required for LWAs.
When clay is used to produce synthetic aggregate, a suitable raw material is heated to a sufficiently high temperature to melt it into a viscous, pyroplastic mass. The entrapped gases in the viscous mass cause the expansion or bloating of the mass, and subsequently, a porous structure with low density results upon cooling. Some basic requirements for raw material to be used in synthetic aggregate production are: the raw materials should melt at a temperature not exceeding 1300° C.; the raw materials should contain sufficient gas-forming ingredients for bloating, and the gases should be evolved at the temperature at which melting occurs; and the viscosity of the melt should neither be too high nor too low (a viscosity that is too high results in thick wall formation in the finished product, whereas a viscosity that is too low results in insufficient bloating).
The melting temperature is mainly controlled by the mineralogical composition of the raw material. When clay is subjected to heat treatment, liquefaction starts with the melting of minerals requiring the lowest temperature. Once such minerals begin to melt, they act as a fluxing or dissolving agent and tend to lower the melting point of the minerals in contact with them. Some low-temperature fluxing minerals include sodium and potassium salts, and silicates. Feldspar provides fluxing action over a wide range of temperatures.
The constituents necessary for gas evolution at a temperature coincident with the molten state are reported to be pyrite, hematite, and dolomite. The various gases generated from these minerals or other sources that bloat the viscous mass include oxygen, sulfur dioxide, sulfur trioxide, carbon monoxide and entrapped air. The evolution of oxygen from the dissociation of ferric oxide at high temperatures is considered to be very significant in the bloating of clays.
While the above is exemplary of a viable synthetic aggregate composition, the above utilizes only one source of waste. Moreover, it is highly specific to a small number of industries, i.e., the semiconductor industry, and as a corollary, in some instances, to a specific region. Many industrialized economies and nations worldwide exist that have limited natural resources and several streams of wastes. In light of the above, it would be a benefit to the construction industry to develop a process for producing a synthetic aggregate that can incorporate a plurality of waste sources in the composition thereof with similar performance to natural aggregates. Such a process allows for recycling of waste, which will assist in substantially reducing the environmental impact while meeting aggregate demands.
Thus, a process using multiple waste streams to manufacture synthetic lightweight aggregate solving the aforementioned problems is desired.